This amp has all germanium transistors made by Sanyo Japan and
Ates Italy, that may be part of a repair of this amp.
The power transistors are 2SB424 up to AD142 or 2SB 426 depending on the output
power and the other transistors
are low power / low
noise Sanyo 2SBxxx. The Ates transistors do their job very well in this amp.
Other types of final power transistors were used also.
At the time this was a relatively cheap amplifier, costing less than its
competitors with great well known brand names like Luxman.

Below the basics diagram of
this amplifier, the actual schematic diagram is slightly different,
this is shown to point out the use of a driver transformer and a OTL output
stage.

Another example of this kind of schematic diagram here:
V

Another example of this kind of schematic diagram
here: (part from tonegems.com) V

This amplifier may have been sold under
different brand names. If you know another brandname, let me know.

Here you can see the internals of the amp, its well built following traditional
practises and the
driver transformers have a prominent place inside.
All parts are still working and in good order after so many years, only the
electrolytic caps need to be replaced.

Replacing the power main and all signal path capacitors, the modern
ones are of higher voltage, better quality and a lot smaller.
Using an extra capacitor in the main power section and changing the chassis
wiring into "star" topology (positive + power terminal to chassis),
this makes the amp better in low frequency performance and low noise.

Sound:

The sound is remarkably good for such an early
vintage transistor amplifier, notwithstanding the old noisy
germanium transistors in the pre-stages,
this is a good quality
10 Watts/ch capable power aplifier.

Modifications:

Replace all electrolytic capacitors in the power supply and
power rail as shown here and all in the signal path, use SQ or LL versions on
critical places.
Tighten all screws, as they
may have loosening after many years of service. Replace all parts that
are broken or worn out.
Check all voltages and currents in idling and dynamic working situation.

The new speaker connection terminals, the old ones were worn out.
V

Below a newer version of this
Chicago amplifier. It has all silicon transistors inside and has a complementary
power stage, the power rail is still assymetric.
Also a 5 way input selector + a European DIN tape input and a headphones jack is added. The setup of
this amp is quite the same as you can see.

The power transistors: 2SD325 drivers and 2SD313
+ 2SB507 final stage.